In a letter to her mother, Margaret wrote: 'I am going back today to clear up some more of your room. Keeping the letters for you to sort later.'

Shawcross writes: 'On the princess's orders, large black bags of papers were taken away for destruction rather than for ultimate consignment to the Royal Archives. There is no record of just what was thus lost but Princess Margaret later told Lady Penn (who was related by marriage to Queen Elizabeth's private secretary, Arthur Penn) that among the papers she had destroyed were letters from the Princess of Wales to Queen Elizabeth - because they were so private.

'No doubt Princess Margaret felt that she was protecting her mother and other members of the family. It was understandable, although regrettable from a historical viewpoint.'

The Queen Mother was thought to be very close to both Charles and Diana but, the book says, reacted with 'utter abhorrence' over Diana's decision to 'wash the dirty linen in public' by disclosing details of the breakdown of her marriage.

She also 'regretted' Charles' decision to discuss his private life on a television programme where he admitted he had been unfaithful to Diana.